Ear temperature monitor and method of temperature measurement

ABSTRACT

A continuous body core temperature monitor comprises a pliable ear plug that conforms to the shape of an ear canal and incorporates a temperature sensor that is clamped between the plug and the ear canal wall. The external surface of the plug is connected to an external temperature sensor and a heating element that compensate for a heat lost from the ear canal to the environment by maintaining the temperature gradient between the temperature sensor and the heating element close to zero.

This application is a continuation of application Ser. No. 09/927,179,filed on Aug. 8, 2001 (now U.S. Pat. No. 6,773,405), which claims thebenefit of U.S. provisional application Ser. No. 60/233,104, filed onSep. 15, 2000 (abandoned), the disclosures of which are fullyincorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a method of monitoring temperature of a humanbody and devices for achieving same and, more particularly, to such amethod and device which monitors the internal core temperature of aperson undergoing continuous medical observation.

DESCRIPTION OF PRIOR ART

Frequently, during surgical and other medical procedures related tohumans and animals, there is a need for continuous monitoring of thebody core temperature. Core temperature here means temperature of bloodflowing around the brain and other vital internal organs. It has beenrecognized long time ago that the core temperature is an accurateparameter for assessing the physiological functions and metabolicactivity of a body.

Traditionally, there are several known devices for continuous assessingbody temperature of a patient. All these devices primarily differ by themeasurement site. Specifically, they are 1) an esophageal probe, 2) arectal probe, 3) skin temperature probes, and 4) an intermittent instantear thermometers, often called tympanic. The last device presently cannot provide a continuous monitoring. The first two devices yieldaccuracy well acceptable for the diagnostic and monitoring purposes andaccount for the majority of present temperature recordings. Thesetraditional devices are invasive, may require sterile probes(esophageal), often inconvenient and, as a rule, not acceptable forpatients outside the operating rooms. A skin temperature monitoring isused sporadically as it is more influenced by the ambient temperature.The need for an easy, inexpensive, accurate, and comfortable way ofcontinuous temperature monitoring is substantial.

It has been recognized long time ago that the tympanic region of the earcanal follows the body core temperature with high fidelity. The regionincludes the tympanic membrane and the adjacent walls of the ear canal.This premise has been the basis for the tympanic thermometers, includingboth the contact and non-contact (infrared) types. An example of acontact transducer is a miniature thermistor (produced, for example, byVital Signs, Inc.) that is positioned directly on the surface of atympanic membrane with the connecting wires secured inside the earcanal. Generally, this can be performed only on an anesthetized patientwith a risk of damaging the tympanic membrane and thus is rarely used inmedical practice. Another example is a contact temperature transducerthat is incorporated into an ear plug (U.S. Pat. No. 3,274,994).Examples of continuous noncontact optical infrared probes are given inU.S. Pat. Nos. 3,282,106 and 3,581,570.

Contact detectors are much simpler than noncontact, but they both sufferfrom the same effect—difficulty of a reliable placement inside the earcanal. Placement of a contact temperature transducer inside the earcanal without a reliable securing of it at any specific position maycause a high inaccuracy in measurement, due to unpredictable effects ofthe ambient temperature and placement technique of the probe. An attemptto incorporate a temperature transducer into an ear plug similar to ahearing aid device is exemplified by U.S. Pat. No. 5,333,622 issued toCasali, et al. Yet, the teaching does not resolve the accuracy problemdue to heat loss. Besides, such a probe requires an individual tailoringof its shape. It should be noted that besides a temperature measurement,there are some other types of measurements that may require a secureadaptive positioning of a transducer inside a body cavity.

Therefore, it is a goal of this invention to produce a sensing devicethat can be positioned securely and reliably in a body cavity;

Another goal of the invention is to make an ear temperature transducerwith a contact probe that is automatically secured at an ear canal wall;

It is another goal of this invention to produce an ear temperaturetransducer that tracks the core temperature with high fidelity;

It is another goal of this invention to make an ear temperaturetransducer that is less influenced by the ambient temperature;

It is another goal to provide an ear temperature transducer that doesn'tcause a discomfort for a patient and can remain in the ear canal for aprolonged time;

SUMMARY OF THE INVENTION

The goals of this invention is achieved by the novel ear temperaturedetector. The detector is comprised of an ear plug carrying thetemperature sensing device wherein the sensing device is characterizedby its increased thermal coupling to a wall of an ear canal anddecreased coupling to the environment. This is attained by pre-shapingthe plug into a smaller size and allowing to change its shape upon theinsertion, until the sensing device is clamped between the plug and theskin. To correct for a thermal gradient across the ear plug, the plughas low thermal conductivity and its external temperature is monitored.Alternatively, temperature of the external portion of the plug isactively controlled by a heater attached to the plug. The heater forms athermal shield around the temperature sensing device, thus negating athermal gradient across the plug.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a temperature detector inserted into an ear and secured ona helix.

FIG. 2 is a cross-sectional view of an ear temperature detector in astorage state

FIG. 3 is a cross-sectional view of a temperature detector in expandedstate

FIG. 4 is a temperature detector with the electronic module inside theplug

FIG. 5 is a fork version of an ear plug

FIG. 6 shows a block diagram of a temperature monitor

FIG. 7 depicts a block diagram of a temperature monitor with anadditional heater

FIG. 8 is an electrical circuit diagram of a controlled heater withthermistor sensors.

FIG. 9 is a radio telemetry version of a temperature monitor

FIG. 10 depicts a practical assembly of an ear temperature detector

FIG. 11 shows a tympanic sensor with a compensating heater

FIG. 12 illustrates a cross-sectional view of a surface temperaturesensor

FIG. 13 is an electrical circuit diagram of a controlled heater with athermocouple sensor

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention describes a device and method for obtaininginformation from a body cavity. At least three essential elements arerequired for this invention to work: a temperature transducer, a thermalinsulator, and an external temperature sensor.

A preferred embodiment is illustrated herewith by showing how this canbe accomplished with improved accuracy when the information istemperature and the body cavity is an ear canal of a human or otheranimal. The major task for accomplishing the stated goals is to increasea thermal coupling between the ear canal walls and a temperaturetransducer, while minimizing such coupling to the external environment.FIG. 1 illustrates an ear plug 4 that is inserted into ear 1, but notreaching the tympanic membrane 3. Temperature transducer 5 is clampedbetween plug 4 and ear canal walls 2. The transducer is connected to theelectronic module 8 via wires 9. There may be more than one transducerattached to the plug, but for the preferred embodiment just one is asufficient example. The module is positioned in the external supportingdisk 7 that contains external temperature sensor 21. The entire assemblymay be secured on ear 1 by carrier clamp 6 that has shape suitable forencircling the helix of an ear. Naturally, other conventional methods ofsecuring may work as well. Since the ear canal wall temperature is closeto that of tympanic membrane 3, it is assumed that transducer 5 canmonitor the tympanic temperature, unless plug 4 and wires 9 sink asignificant portion of thermal energy from the transducer, resulting inerroneous temperature measurement. The position of transducer 5 insidethe ear canal has to be consistent and always between plug 4 and walls2. The output signal is measured via conductors 70.

To achieve the desired results, transducer 5 is attached to a specificportion of plug 4. That portion preferably should be at the distal endof the plug that would be inserted into a body cavity, such is an earcanal. FIG. 2 shows plug 4 in a storage stage, that is, before it isinserted into an ear. The plug has two ends—base II and tip 12. The baseis attached to an external enclosure. The enclosure is in form of disk 7that may have a protruding pin 45 inside the plug for better mechanicaland thermal coupling between disk 7 and plug 4. In a storage state orjust prior the insertion into the ear canal, tip 12 is compressed to asize that is smaller than the inner dimension of the ear canal. Toretain such reduced shape for a long time, the tip may he inserted intostorage sleeve 10 that provides a constraining compression. The sleevemay be a plastic tube. Plug 4 is fabricated of pliable material that maybe collapsed when squashed (compressed) and recover its original shape(expand) when external pressure is released. The plug serves as athermal insulator. Its thermal conductivity should be minimal, thusfoams are the best choice of the material. An example of such a materialis water-born hydrophilic foam. The foam should not have a significantdimensional memory so that it returns to the original expanded shapeafter prolonged storage in the collapsed (compressed) shape.

For a better thermal speed response, temperature transducer 5 is securedon the surface of tip 12. The transducer should have a small size andmay be of any suitable design—thermistor, thermocouple, semiconductor,etc. Wires 9 should sink out as little heat as possible, thus they needlo be fabricated as thin as practical and should have an extended lengthinside or on the surface of plug 4. To increase the length, wires 9 maybe formed into loop 13 that is positioned between transducer 5 andelectronic module 8, regardless of position of the module (explainedbelow).

Before insertion of the plug into an ear canal, sleeve 10 is removed anddiscarded. Shape of tip 12 slowly returns to that which wasprefabricated before the installation of sleeve 10. Alternatively, tip12 may be squashed by an operator just before the insertion. The rate ofthe shape recovery should be sufficiently slow to allow enough time forthe insertion of plug 4 into an ear canal. Practically, the shaperecovery time should be greater than 3 seconds. After the collapsed tip12 is inserted into an ear canal, its continuous shape recovery forcesplug 4 to conform with the shape of an ear canal. The expansion of plug4 stops when it completely fills up the adjacent ear canal volume. Thisallows transducer 5 to be forcibly compressed against ear canal wall 2,while still being electrically connected to electronic module 8 viawires 9, as shown in FIG. 1.

Electronic module 8 may contain the amplifier, power supply, signalconditioner, transmitter and other components, or it may be a simpleconnecting device. In some embodiments, module 8 may be positioneddirectly inside plug 4 as shown in FIG. 4. In this case, the size ofmodule 48 should be small enough to allow compression of tip 12 beforethe insertion. Module 8 may be used for many other purposes, in additionto or instead of measuring temperature. An example is generating sound.In that case, opening 44 in plug 4 may be required for better soundcoupling to the lympanic membrane.

It should be stressed that an ear canal is just an example of anapplication and the identical concept of an expandable plug with anattached transducer can be used for producing an insert for other bodycavities, for example, nasal. Further, there maybe other thantemperature transducers attached to the plug, for example acoustic.

Another possible embodiment of plug 4 is shown in FIG. 5 where the plugis made in shape of flexible fork 36 having a spring action. The end ofthe fork is squeezed by fingers 35 before the insertion and let goafter. The fork has arm 37 that carries transducer 5. After the fork isreleased, it expands so that its arm 37 compresses transducer 5 againstear canal wall 2. To improve thermal separation of transducer 5 from theoutside, the fork may be supplied with insulator 38. Other components,like the wires, the loop, the electronic module, etc, are not shown inFIG. 5.

The expanded plug 4 performs an important function—positioning andclamping transducer 5 on an ear canal wall surface. The other criticalfunction—minimizing effects of the ambient temperature may beaccomplished by at least two methods. One method is a mathematicalcorrection and the other is an active compensation. The method of amathematical correction of an error is performed by the use of anadditional ambient temperature sensor that is positioned either directlyon disk 7 as external sensor 21 (FIGS. 2 and 3), or in/on the externalmonitor 16 as ambient sensor 20 (FIG. 6). Note that for this method,heater 14 is not required and only one sensing device—either ambientsensor 20 or external sensor 21 is needed. Disk 7 of an ear device isconnected to monitor 16 via cable 15 (FIGS. 6 and 7). Monitor 16 maycontain signal processor 17, power supply 18, display 19 and othercomponents. Ambient sensor's 20 or external sensor's 21 signal isprocessed and used to correct for errors in the ear temperaturemeasurement. The degree of correction needs to be establishedexperimentally for a particular plug design. The corrected bodytemperature t_(b) may be determined through a temperature gradient, forexample, as:t _(b) =t _(s)+μ(t _(s) −t _(a))  (1)where μ is the experimental constant, t_(a) is the temperature measuredby ambient sensor 20 or external sensor 21 and t_(s) is the reading ofear temperature transducer 5.

The above method of error correction, however, has it's limitations. Oneis the uncertainty in the value of constant μ. Another limitation is theuse of ambient sensor 20. Having ambient sensor 20 placed at monitor 16,makes the mathematical correction less effective when, for example, thepatient is laying on the ear which is being monitored and thus havingthe external ear temperature significantly different from that ofambient monitored by sensor 20.

A more effective method of the error reduction is an active heat losscompensation that is shown in FIG. 7. It is based on forming a thermalshield around temperature transducer 5. Disk 7 carries heater 14 andexternal temperature sensor 21, positioned on or near heater 14 with agood thermal coupling between them. Note that disk 7 is located outsideof the ear canal, directly at it's opening. Heater 14 also may be seeingin FIGS. 2, 3 and 4. The heater controller, that is positioned eitherinside disk 7 or in monitor 16, as shown in FIG. 7, receives signal fromexternal sensor 21 and controls temperature of heater 14 to a requiredlevel, that should be close to the actual body temperature as monitoredby transducer 5. Thus, heater 14 minimizes temperature gradient betweentemperature transducer 5 and heater 14. It acts as a thermal shieldbetween temperature transducer 5 and the environment. Circuit diagram ofFIG. 8 further illustrates this method. A reference point for the heatercontrol is provided by temperature transducer 5 positioned inside theear canal and compressed by plug 4 to the ear canal wall. Bothtemperature transducer 5 and external sensor 21 are connected to theWheatstone bridge circuit with two pull-up resistors 30 and 31. Thermalcoupling between transducer 5 and the ear canal walls needs to be muchbetter than between temperature transducer 5 and the externalcomponents, that is, external sensor 21 and heater 14. This is primarilyaccomplished by the use of thermally insulating plug 4. An excessivethermal coupling between temperature transducer 5 and external sensor 21may result in undesirable instability of the control circuit. Erroramplifier 32 compares the output signals from temperature transducer 5and external sensor 21 and controls heater controller 22, that in turn,via conductors 33, adjusts electric power to heater 14. This circuitmaintains temperature of heater 14 close to that of temperaturetransducer 5. This results in a negligible heat transfer through plug 4and elimination of the error in temperature measured by temperaturetransducer 5. Turning again to Eq. 1, we can see that with the activeheating of the above thermal shield method, temperatures at both sidesof plug 4 equalize: t_(a)≈t_(s) and thus value of μ become irrelevant,so that t_(b)=t_(s). In other words, transducer 5 now directly measurestemperature of the body with no influence of the ambient temperature.

As a variant of FIG. 8, FIG. 13 illustrates use of a thermocoupletemperature transducer having two dissimilar conductors 100 and 101. Athermocouple has two junctions, hot junction 102 and cold junction 103.Hot junction 102 is positioned inside the body cavity at one end of plug4, while cold junction 103 is thermally attached to heater 14 andexternal sensor 21 near the other end of plug 4.

Heater controller 22 receives signal from thermocouple amplifier 32 andoperates such as to bring thermocouple output voltage 105 close to zero.This will establish a minimal thermal gradient across plug 4 so thatexternal sensor 21 indicates the internal body temperature.

The use of cable 15 as shown in FIGS. 6 and 7 may not be desirable, asit may restrict movement of a patient. The cable can be eliminated ifdisk 7 carries transmitter 24 and power source 25, as illustrated inFIG. 9. Accordingly, monitor 18 needs to contain antenna 27 and receiver23. The link between the patient and the monitor may be via radio waves26, or optical (both involve electromagnetic radiation). Alternatively,transmitter 24 and/or power source 25 can be located outside of disk 7,but in that case, an intermediate packaging for these components (notshown) would be required. It should be noted, that in the wirelesscommunication with the monitor, method of a passive error correction ispreferable, so that transmitter 24 will send information concerning blottransducer 5 and external sensor 21.

A practical way to produce an ear temperature monitoring device with athermal shield is shown in FIG. 10. Reusable cup 39 may containelectronic module 8, cable 15, second contacts 29, heater 14, andexternal sensor 21. A detachable part is disposable insert 41 thatcontains plate 40, plug 4, and transducer 5 attached via wires 9 tofirst contacts 28. Before operation, insert 41 is moved in direction 42to mate with cup 39. Both cup 39 and insert 41 are engaged and retainedtogether during the temperature monitoring with the help of lock 43.Contacts 28 and 29 provide connection between wires 9 and electronicmodule 8. After the monitoring in completed, disposable insert 41 may bedetached from cup 39 and discarded.

OTHER EMBODIMENTS

A thermal shield method similar to one shown in the preferred embodimentcan be employed to reduce effects of the environment with other types ofthe medical temperature sensors. The general operating principle isbasically the same as described above. FIG. 11 shows an example of atemperature transducer 52 that is directly attached to tympanic membrane3 of ear 1. This embodiment does not necessarily require an expandingplug 4 that has been shown in the prior illustrations. Wires 9 passthrough or near heating insert 50 that is inserted into ear opening 51.The heating insert contains external sensor 21 and thermally attached toit heater 14, whose temperature is controlled to approach that measuredby transducer 52. As above, wires should be as thin as practical andheater 14 should be thermally de-coupled from transducer 52. Since thethermal gradient across wires 9 between transducer 52 and externalsensor 21 becomes small, effects of the ambient temperature also becomesmall, while the accuracy of measurement improves.

Another embodiment of the same thermal shield method is depicted in FIG.12. This is a surface temperature measuring device that can measure a“deep” (subcutaneous) body temperature. The device is comprised ofhousing 55 secured to skin 60 or another surface of a subject, skintemperature sensor 56, heater 14, thermal insulator 58, secondtemperature sensor 57, wires 9, and cable 59. Housing 55 is formed frommetal, as represented by the cross-section lines. Note that wires 9 passthrough insulator 58 and through or near heater 14. Insulator 58 can bea body of polymer foam or even an air gap. In operation, the temperatureof second temperature sensor 57 is controlled to approach that of skintemperature sensor 56, by providing thermal energy to heater 14. Thisforms a thermal shield above skin temperature sensor 56 and minimizesheat loss from skin 60 and, subsequently, to an improved accuracy intemperature measurement. In most practical cases, for an acceptableaccuracy, a typical temperature difference between sensors 56 and 57should be no greater than 2° C. and preferably equal to zero.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made without departing from the invention inits broader aspects, and, therefore, the aim in the appended claims isto cover all such changes and modifications as fall within the truespirit and scope of the invention.

1. A method for continuous monitoring of the internal temperature of asubject, comprising: positioning a first temperature sensor relative toa housing fabricated from metal having high thermal conductivity;thermally attaching a heater to a second temperature sensor and to thehousing, the second temperature sensor being positioned within thehousing; thermally insulating the first temperature sensor from thehousing, the heater and the second temperature sensor; securing thehousing on a surface of the subject; measuring the temperature of thesurface using the first temperature sensor; generating heat with theheater at a rate that minimizes a temperature gradient between the firstand second temperature sensors; and computing an internal temperature ofthe subject using signals from the first and second temperature sensors.2. A method for continuous monitoring of the internal temperature of asubject, comprising: positioning a first temperature sensor relative toa housing fabricated from metal having high thermal conductivity;attaching a second temperature sensor to the housing; thermallyinsulating the first temperature sensor from the housing and from thesecond temperature sensor; securing the housing on a surface of thesubject; measuring the temperature of the surface using the firsttemperature sensor; and computing an internal temperature of the subjectusing signals from the first and second temperature sensors.
 3. Themethod of claim 2, where computing the internal temperature furthercomprises determining a temperature gradient between the first andsecond temperature sensors and subsequently multiplying the gradient byan experimental constant.
 4. A temperature sensing device for monitoringan internal temperature of a subject, comprising: a housing configuredto be in contact with a surface of the subject; a first temperaturesensor thermally insulated from said housing and configured to bethermally attached to the subject for detecting a surface temperature ofthe subject, wherein said housing is fabricated from metal having highthermal conductivity; a second temperature sensor thermally coupled tosaid housing; a thermal insulator positioned between said firsttemperature sensor and said second temperature sensor; and an electronicmodule electronically connected to said first temperature sensor andsaid second temperature sensor.
 5. The device of claim 4, furthercomprising a heater thermally coupled to said housing and electricallycoupled to said electronic module.
 6. The device of claim 4, whereinsaid thermal insulator further comprises an air gap between said firsttemperature sensor and said second temperature sensor.
 7. The device ofclaim 4, wherein said electronic module further comprises a datatransmitter for broadcasting responses of said first temperature sensorand said second temperature sensor.
 8. The device of claim 4 furthercomprising an external monitor coupled to said electronic module andcontaining a power supply and signal processor for computation of theinternal body temperature of the subject.